Research On 2?20GHz Small Step Low Phase Noise Frequency Synthesizer

As the core component of wireless electronic products,frequency synthesizer provides high quality local frequency for receiver and transmitter.With the rapid development of ultra-wideband transceiver front-end,the frequency synthesizer is required to have the advantages of ultra-wideband,fine frequency resolution,low phase noise,low spurious and so on.In this paper,based on the hybrid frequency synthesis technology,a 2?20GHz frequency synthesizer with ultra wideband fine frequency resolution and low phase noise is designed by using the structure of dual phase-locked loop.Firstly,cascaded mixer phase-locked loop is designed to achieve the performance of ultra wideband and low phase noise.The phase-locked loop of the cascade mixer adopts the structure of two-stage mixing in the loop,which can make the division ratio in the loop fixed to 1 in the whole frequency range of the phase-locked loop,and eliminate the influence of the division ratio in the loop on the phase noise.The direct frequency synthesis technology is used to expand the output frequency and realize the frequency range of 2?20GHz.By means of switch switching,the phase-locked loop chip self locking is used to solve the frequency preset problem.Secondly,a reference local oscillator phase-locked loop is designed to provide small step reference frequency and mixing local oscillator frequency for the cascaded mixer phase-locked loop.The reference local oscillator phase-locked loop uses DDS to drive the phase-locked loop,which not only achieves the performance of fine frequency resolution,but also effectively suppresses the output spurious of DDS.At the same time,the phase noise is reduced by the first-order mixing structure in the loop.Then,the parameter configuration algorithm of dual phase-locked loop is researched and designed.The core of the algorithm is the structure of the dual phase-locked loop and relationship between the parameters.According to different output frequency values of frequency synthesizer,the parameters can be quickly responded.Finally,this paper selects the key components and completes the circuit hardware design.The test results show that the output frequency range of the frequency synthesizer designed in this paper covers 2?20GHz,and it can realize the frequency step of 1Hz.When the output frequency is 20 GHz,the phase noise can reach-98.4 d Bc/Hz @ 1k Hz,and the overall output spurious suppression is greater than-70 dBc,which meets the index requirements.